The jury's verdict is guilty. 

The ancestor's wisdom is astonishing.
My sister's opinion is important.
I have three weeks of vacations.
Carlos and Miguel's cellphone is very expensive.
My girlfriend's sister is my bestfriend best friend.
Anna and Nicole's project is very good.
My mother's sister has five children.
The laboratory's tools are on the wall. 
The people's lives is are not good.

Perfect except that they don't sound like prayers and I see a couple of small errors. 

When i can use 's

Hi I have some problems with these prayers, I do not know if they are correct.

The jury's verdict is guilty.
The ancestor's wisdom is astonishing.
My sister's opinion is important.
I have three weeks of vacations.
Carlos' (or Carlo's depending on what the name is, Carlo or Carlos) and Miguel's cellphone is very expensive.
My girlfriend's sister is my best friend (as far as I know this is not a compound word but two separate words).
Anna and Nicole's project is very good.
My mother's sister has five children.
The laboratory's tools are on the wall.
The people's live is not good. People does not agree with live and is. This sentence would need to be one of: The people's lives are not good. OR The people's life is not good.

:)